A coal-fired power plant has a flue-gas desulfurization unit for treating flue gas so as to remove sulfur oxides (SOx) generated by combustion of coal in a boiler. Various types of wet- or dry-type flue gas desulfurization units have been proposed, but a wet coal gypsum limestone-gypsum method is conventionally often used because cheap limestone can be used as a desulfurizing agent and a relatively high sulfur removal ratio can be achieved (Patent Literature 1).
The limestone-gypsum method is performed by bringing flue gas into gas-liquid contact with an absorbent in slurry state containing finely-powdered limestone to allow the absorbent to absorb SO2 gas contained in the flue gas. Then, SO2 is fixed as gypsum in the absorbent by a reaction represented by the following formula 1 to remove sulfur oxides.SO2+CaCO3+½O2+H2O→CaSO4.2H2O+CO2  [Formula 1]
In the limestone-gypsum method, the gas-liquid contact between the absorbent and the flue gas can be efficiently performed using, for example, a reactor equipped with a sparger pipe (a jet-bubbling reactor). More specifically, the reactor is filled with the absorbent so that the tip of the sparger pipe is submerged therein, and then the flue gas is discharged from the tip of the sparger pipe into the absorbent. By doing so, the flue gas rises as fine bubbles in the absorbent, during which gas-liquid contact between the absorbent and the flue gas is efficiently performed at the interface of each bubble.
Sulfur oxides absorbed by the gas-liquid contact are oxidized to sulfuric acid and at the same time, it is neutralized by the absorbent in which limestone is dissolved so that gypsum is produced. The slurry containing the thus produced gypsum is extracted from the reactor, sent to a solid-liquid separation means such as a centrifugal separator, and separated into granular gypsum and a liquid fraction. Part of the separated liquid fraction is returned to the flue-gas desulfurization unit, and the rest is sent to a wastewater treatment unit and further treated.